Novel approaches are proposed to improve the quality of stored platelets at any storage interval and to extend the duration of platelet storage. A broad spectrum of storage conditions will be evaluated. Two major hypotheses will be tested that may explain the progressive loss of platelet viability during storage: 1) the metabolic needs of platelets are not being met during storage; and/or 2) platelets are progressively activated during the collection, processing, and storage of platelets leading to rapid platelet removal following transfusion. Studies are planned to test the post-transfusion viability and function of platelets stored in solutions designed to meet the metabolic needs of platelets with or without the addition of activation inhibitors. Long-term platelet storage will be evaluated using agents that have been documented to maintain the integrity of a variety of cells and tissues during cryopreservation and/or freeze-drying (lyophilization). Other approaches involve warming platelets to 37 C for one hour prior to transfusion to facilitate repair of any reversible storage-induced lesions that previously resulted in the immediate removal of a damaged population of platelets from circulation. We will also collect and store platelets from apheresis donors who have been previously given thrombopoietin (MGDF) to increase their platelet counts. It is hypothesized that platelets harvested from these MGDF-stimulated donors will contain a population of younger platelets that may be more resistant to injury during both liquid storage, and/or during cryopreservation or freeze-drying. Several new instruments and assays that may better predict post- transfusion platelet viability and function than the currently-available in vitro assays will be tested for their correlation with post- transfusion platelet recovery, survival, and bleeding time measurements.